For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index.

The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20 cm. The focal length of lenses are

A concave mirror of focal length $$f$$₁ is placed at a distance of d from a convex lens of focal length $$f$$₂. A beam of light coming from infinity and falling on this convex lens $$-$$ concave mirror combination returns to infinity. The distance d must equal

When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index